Probably the most insultable male body part. A penis insult by a man is good, by a woman, better, and when wielded by a girlfriend / wife (current or ex-) it is a formidable power. Of course, it is not necessary to have actually seen the penis in question, but if you have seen it, remember to add, "and I know what I'm talking about," at the end of the insult.
Now, girlfriends and wives, I must remind you - just because you love him doesn't mean that you shouldn't comment about how small his penis is in mixed company. Of course, any man who truly loves you will understand the joy it brings you to insult him and will in fact encourage you to insult his penis in social gatherings.
Naturally, any occasion is a penis-insult occasion, but some frequently occuring situations are summed up here:

For men - the breast is an object of desire. It is the first part of the woman they see and the first one they talk about. For women, the breast is a neurosis. Just as men are insecure about their penises, women are insecure about their breasts. That is why women can be subdivided into two groups:

iii) Other body parts

Basically, there is no reason to insult any body part other than the penis or breast, as these are the most prominent (or not) parts. There are other body areas worth insulting, and these are, in order of importance: